Pine and cedar are DEADLY to small animals as it gets in their lungs and causes breathing problems. They really should never be sold. Carefresh is a better option. It is worse for smaller ones, though rabbits and guinea pigs will still suffer problems from them.
For herps (reptiles) there is a huge amount stuff you would need to consider. What I do know is bad is that sand they sell for like geckos and beardies. They ingest it, get impacted, and die. :( So sand is a big no-no for reptiles (except for sand boas, but even then, the sand they sell at petstores just tends to be horrible). I use coco husk/fiber for my desert tarantula and she loves it. Also, tarantulas should never be fed pinky mice. The calcium makes it hard for them to shed and sometimes die.
There is no good pet for kids. Just straight up tell a parent that if the kids get's bored, and they probably will, they (the parent) have to want to take care of that animal. And there is at least nothing wrong with going home and talking about it and researching something before they come back (you can put animals on hold). No need to impulse buy.
I never understand why it is "bad" when you tell someone they need the bigger cage or better bedding for an animal... You are upselling! They end up spending more money when they buy the better products! If your manager gets on you about that, you might try pointing that out.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?